It is important for a wind turbine to have advance knowledge to the condition of the wind which will shortly arrive at the turbine. Such knowledge gives the turbine controller sufficient time to adjust operating parameters, such as blade pitch angle or rotor speed, to match the oncoming conditions. This may be done for a variety of reasons. At lower wind speeds it is important to maximize the energy that can be extracted by the wind by setting parameters such as blade pitch angle to an optimum position. At higher wind speeds it is important to adjust turbine parameters to avoid operation under conditions which might lead to damage. Wind turbines have a rated power and when this power output is exceeded, the blade pitch angle and other operating parameters will be adjusted to reduce the amount of energy that is extracted from the wind. Wind turbines also need to be designed to withstand extreme operating conditions. Typically, these extreme conditions are rare single events or a small number of cumulative events which cause large, often unbalanced, loads on the wind turbine and will damage the turbine or reduce the overall lifetime of the turbine components, such as the blades or the gearbox by a significant amount.
Wind turbines are required to meet design parameters relating to extreme conditions as set out in International Standard IEC 61400-1 3rd Edition 2005. Section 6.3.2 of the standard defines a range of extreme wind conditions including extreme wind speed, extreme turbulence, and extreme gusts. In order to meet these requirements, wind turbines are required to be considerably over-engineered for normal use which greatly increases the amount of material used in turbine components such as the blades, the gearbox, and tower. This in turn increases the cost of wind turbine installation and the cost of energy generation.
Many proposals have been made for determining advance wind conditions. Generally these include sensors placed on the turbine which observe the upstream wind. One example is disclosed in EP-A-0970308 which discloses the use of a Lidar or similar remote sensing apparatus, mounted on the nacelle on the wind turbine, and sensing conditions several rotor diameters upstream of the turbine. Based on the sensed conditions the controller, which may be onboard the turbine or may be a separate wind power plant controller, can instruct an individual turbine or group of turbines to change their operating parameters before the sensed wind conditions arrive at the turbine. Another example is disclosed in US-A-20060140764, in which the Lidar is mounted in the rotor hub and has a plurality of look directions that are inclined away from the rotational axis of the hub so that rotation of the hub ensures a conical scan of the upstream air by the beam. The multiple look directions may also be achieved by using a number of dedicated Lidar systems and/or by using multiplexed Lidars or a beam splitter.
The known art provides a number of examples of advance sensing of wind conditions at a wind turbine. However, none of the art enables the detection of an extreme change in operating gusts and so none is able to protect the turbine from the consequence of such an extreme gust. The present invention aims to address this deficiency in the prior art.
Accordingly, the invention provides a control system for a wind turbine, comprising a sensing device mounted on the wind turbine to sense wind speed at a position upwind of the wind turbine; a controller for receiving and processing signals from the sensing device to detect an extreme operating gust upwind of the wind turbine, and for generating one or more control signals for varying an operating parameter of the wind turbine in response to the detected extreme operating gust, wherein the controller comprises a differentiator for differentiating a measure of the sensed wind speed.
The invention also provides a method of controlling a wind turbine. Comprising sensing wind speed at a position upwind of the wind turbine using a sensing device mounted on the turbine; receiving and processing signals from the sensing device at a controller to detect an extreme operating gust upwind of the wind turbine, wherein processing the signals comprises differentiating a measure of the sensed wind speed; and generating one or more control signals for varying an operating parameter of the wind turbine in response to the detected extreme operating gust.
Embodiments of the invention have the advantage that by differentiation of a suitable wind parameter, the onset of an extreme operating gust can be detected This enables the system controller to determine whether there is an extreme operating gust as defined by IEC 61400-1 discussed below and, thus, whether evasive action need be taken.
In one embodiment of the invention a filter may determine whether the output of the differentiator exceeds a predetermined value for a predetermined period of time. This has the advantage that instantaneous changes, which may be caused by turbulence, are not flagged as extreme events and an extreme event is only signalled when the detected change is maintained for a period of time.
The predetermined value may be dependent on the magnitude of the sensed wind speed. This is advantageous as the higher the wind speed, the lower the increase in velocity required to cause over-loading and damage. The predetermined period of time may be up to 3 seconds but typically 1 second.
In another embodiment of the invention, the sensed wind parameter is wind speed, and wind speed signals are resolved into axial, lateral and, optionally, vertical components. The differentiator acts on the axial component to determine the rate of change of axial wind speed. This approach is advantageous as it may provide a more accurate measure of gust speed. Alternatively, the differentiator may act on either the square root of the sum of the squares of lateral and axial wind components, or the square root of the sum of the squares of lateral, axial and vertical wind components.
The sensing device may sense the wind parameter at a plurality of distances upwind of the wind turbine. This has the advantage that changes in the oncoming wind front can be detected so that unnecessary evasive action is not taken when an extreme operating gust detected at a far distance degrades into an event that is not classed as extreme as it approaches the turbine.
In one embodiment, the sensing device is a multiple beam Lidar having multiple range gates.
The control signal generated by the controller in response to a detected extreme operating gust may comprise a turbine shutdown command or de-rate the turbine.
The invention also resides in a wind turbine having a control system as defined above.